It does have an effect when you're using it to make coffee.

I used to reboil the leftover water in my electric kettle all the time to make coffee.

After a while I was wondering why my coffee taste sucks even though it should be the same.

After some goggling, found about using "fresh-boiled water".

Since then I always readd some water to leftoever water before boiling it. Taste of coffee back to normal.